This invention relates generally to the use of dense phase pulverized coal, and more particularly to a pulverized coal fuel injector.
Since the oil embargo of 1974, there has been a major effort to develop energy systems that are not dependent on petroleum-based fuels. This effort is expected to be expanded as a result of the political unrest in the Mideast oil producing countries. There are a number of programs which are directed toward increasing both coal usage in existing markets, and by introducing new premium quality coal-based fuels to markets currently dependent on oil or natural gas. In the American industrial sector, a significant portion of the total boiler capacity in operation in the year 2000 is expected to be composed largely of pre-1980 units. Any substantial near term increase in the use of coal for raising industrial steam will therefore have to be accomplished through conversion of existing boilers currently filled by natural gas or oil.
The application of coal or coal-based fuels to industrial boilers originally designed for gas or oil firing presents several technical challenges to the combustion engineer. Constraints such as carbon conversion efficiency, emissions control, and coal and ash handling must be addressed and resolved. In realizing the conversion of existing oil or gas fired burners, significant effort has been made in the past decade in the fields of coal beneficiation, coal-based alternate fuels, and advanced combustion.
By definition, burners introduce and mix the fuel and combustion air in a boiler or process heater to obtain a stable flame. A key function of most burner designs is to manipulate the inflowing combustion air and fuel streams in such a way as to promote rapid fuel ignition by providing the correct chemical, thermal, and kinetic environment to support combustion of the incoming fuel. Flame stability is normally enhanced by creating static air pressure gradients downstream of the burner exit which intensify fuel/air mixing and promote a small volume of combustion products to recirculate back towards the burner. This recirculating mass entrains a portion of the combusting air stream, the luminous flame front is then stabilized when the burner flame propagation speed matches the combustion air flow regime. Also, burner operation is simplified by stable flames which are insensitive to small random fluctuations in stoichiometric ratio and fuel composition.
Using finely ground cleaned coal allows a combustion process resulting in a clean stream of hot gas. However, while combustion of fine cleaned coal reduces ash, the combustion process can be difficult to control. Combustion must occur in a manner which insures good control of the temperature and the local and overall stoichiometric ratio (the ratio of air to fuel in the mixture) to insure complete combustion, and to avoid formation of nitrogen oxide (NO.sub.x) pollutants. A fuel injector has been developed which is based on the concept of controlling NO.sub.x by maximizing the devolatilization of nitrogenous species in a substoichiometric environment.
The means for injecting the coal fuel into the combustor has an effect on the production of the pollutant NO.sub.x, and several types are well known in the art. For example, U.S. Pat. No. 4,654,001 discloses a burner nozzle for pulverized coal in which mixing members mix fuel passing around and through outer and inner tubular members. The fuel is further divided into an outer fuel-rich stream and an inner fuel-lean stream. The coal-rich stream is ejected as a circular stream, while the air in the inner fuel-lean stream is not available for combustion in the initial combustion region adjacent the burner nozzle.
U.S. Pat. No. 4,497,263 is a burner which receives a stream of coal and air and forms two mixtures: one containing most of the coal and the other containing most of the air. A convergent-divergent discharge tube discharges the fuel, and swirler blades cause the coal and air to intermix and recirculate, resulting in a rich mixture.
U.S. Pat. No. 4,457,241 discloses a low NO.sub.x burner nozzle in which a tubular nozzle has a divergent flow section and a convergent flow section. A flow spreader which is positioned adjacent the outlet end of the nozzle discharges the fuel in an annularly shaped, swirling flow pattern.
Accordingly, it is an object of the present invention to control the formation of NO.sub.x by maximizing the devolatilization of nitrogenous species in a substoichiometric environment through the use of a novel coal injector.
It is a further object of the present invention to promote the production of environmentally benign N.sub.2 rather than NO.sub.x pollutants.
Still a further object of the present invention is to provide an improved and uniform distribution of the fuel within the primary burn zone of a coal combustor and to enhance the effect of improved flame attachment and flame stability.
Yet another object of the present invention is to provide a concentrated and focused fuel rich core to take maximum advantage of the substoichiometric devolatilization principle thus supplementing NO.sub.x reduction.